Serious design effort begins on mini-console project

[mashaffer]

OK, so here is the plan. I will build a quick passive volume control and make sure it works with the amp as currently constructed. That will also give me an idea of the volume levels I can get as a starting point. Then I will rewire the heater for the 12AY7 and change the resistor values as above as a starting point and compare the results. At that point it is just a matter of optimizing R values (easier said than done).

As to the speakers, since I will be using an L-Pad for tone control I think that I will put the crossover in the main cabinet rather than each individual speaker. Then I plan to use 4 wire Speakon connectors for solid connection but still put a 250 ohm or so resistor across the OPT secondaries just in case it is run sans speakers buy a careless youth. I can have a separate set of binding posts for subwoofer connection and or alternative speakers.

I will report back when I get the volume control done.

mike

 

[Kegger]

Any pic's of the amp progress?

 

[mashaffer]

Sadly life's distractions were such that not much has happened since post #71. Hope to start on the volume control tomorrow and dust the cobwebs out for a little spin. If you have any suggestions on biasing a 12AY7 stage I am all ears (well, ears and belly ).

mike

 

[Kegger]

Strictly going by the curves and the tube specs.

Something like 160v at the plate with 2.5v bias and 2.5ma of current.

So to get the bias and current a 1k cathode resistor, and I'd want to
run about a 100k plate load resistor depending on the available supply
voltage which would need to be somewhere around 410v+ or so there.

I'd suspect that to be a good place to run, if it can fit into your setup.

 

[mashaffer]

OK, the simulation I ran the other night was with...

100k plate
1.6k cathode
388V B+

which gave...

230V plate
3.8V cathode
and 2.4mA
gain of 11dB from input to output

Looks like we were aiming for around the same target. .

For comparison I simulated with the 1k you mention resulting in the following...

386V B+
197V plate
2.8V cathode
2.8mA
gain 11.4dB

This is all using 270k for the feedback resistor. Raising it to 310k only increases gain by about 0.5dB. Of course this is all just inexpert simulation but it does give an idea of relative changes. As you say it looks like a relatively linear part of the curves too. Seems like either one ought to provide plenty of voltage swing for iPod/line level input.

Since using your numbers allows me to leave the current Rk in place (never under estimate laziness as a motivating factor ) I will give that a shot first.

Thanks Kegger.

mike

 

[Kegger]

I hadn't realized this was the driver stage, I was thinking intermediate gain stage.
So as to the feedback I would run between (390-470k, would start with 390 here)

I would run the 1k cathode resistor for more current, also yah the plate voltage is
higher then I posted with the feedback resistor bringing in some, which is just fine.

 

[mashaffer]

I ran sims with your new Rfb and got the following

B+ 385
Vp 186
Vg/Ik 2.7
gain 12.7dB

B+ 385
Vp 182
Vg/Ik 2.6
gain 13.2dB

With the 6CG7
B+ 372
Vp 202
Vg/Ik 6.3
gain 4dB

So it looks like 8 to 9 dB increase in gain. I knocked up a volume control with 250k audio pots this morning and gave a listen as a baseline using the existing 6CG7 circuit and my iPod dock. Full gain was no where near full output. I would say it was somewhat more than background music level but still not up to a level that I would choose for any critical listening.

I am not sure that an additional 9dB would be enough. It might be but who knows. Hopefully I can get away without another gain stage. I suppose a 12AX7 could be used but I question how well that would work with this feedback arrangement.

mike

 

[Kegger]

12ax7 won't cut it, not enough current.

And 9db of extra gain is a LOT, you may be just fine with the 12AY7.

 

[mashaffer]

I figured that the AX wouldn't have enough current.

I will give the AY a try. My simulations indicate that with 12AY7 I will need 3V to 3.5V peak to peak at the input to reach full output which I think is unlikely from an iPod dock but there is a lot of hand waving in the creating of the simulation so building the real thing is clearly necessary.

I probably won't get time to try it until the weekend. I will report back. Thanks again.

mike

 

[Kegger]

You can always go to the 12AT7 and or my alternative, the ECC85, as it
has the same specs as the 12AT7 but heater arrangement like the 6CG7.

 

[mashaffer]

BTW, this is what I have in mind for the speakers. This is a first stab at a layout and I have not yet double checked the dimensions. Grid = 1"

The upper driver (above 100Hz) is Mark Audio CHR70 in a sealed sub-enclosure with a labyrinth to minimize reflections. The lower driver (Below 100Hz) is a Dayton 8" Dual coil sub in TL, I intend to use foam core board for first prototype.

mike

 

[mashaffer]

Sorry I haven't gotten back. I am having a great deal of back problems lately and wasn't up to working on this last weekend. Hoping to get to it on my next real day off which is Tuesday.

I just remembered that I have a pair of Cinemag input transformers that I was intending to use on this when I was planning PP with input transformer splitter. IIRC they are 1:1+1 so I could possibly use them to get a 2x gain in the passive preamp if the 12AY7 by itself is note enough.

mike